Cell theory
● All living things have one or more cells
● Cells are the structural and functional units in organisms
● All cells arise or come from existing cells( cell division; meiosis and mitosis)
Type of cells
eukaryotic( possess a nucleus) and prokaryotic( no nucleus)
Type of eukaryotic cells includes; somatic cells and sex cells.
Characteristics of the cell
1. The cell needs nutrients and oxygen
2. Eliminate carbon dioxide and other metabolic wastes from the cell.
3. produces its own energy necessary for its growth.
4. maintains the environment for its survival.
5. show immediate response to the entry of Invaders.
Structure of cell
● Cell membrane
● Cytoplasm
● Nucleus
PLASMA MEMBRANE OR CELL MEMBRANE
The cell membrane is the outer boundary of the cell that separates the cell from its surrounding environment and performs
various functions.
Functions of the cell membrane
1. Protective function, the cell membrane protects the cytoplasm and organelle is present In the cytoplasm.
2. Selective permeability, the cell membrane can allow certain substances in and out of the cell and act as a barrier
for the others.
3. Absorption function, nutrients are absorbed into the cell through the cell membrane.
4. Excretory function, metabolites, and waste products are excreted through the cell membrane.
5. Exchange of gases, Oxygen enters the cell and carbon dioxide exits the cell through the cell membrane.
6. The cell membrane maintains the size and shape of the cell.
Membranes within the cell membrane
1. Membrane lipids
The central lipid layer is a bi-layered structure.
This is formed by a thin film of lipids.
, The characteristic feature of the lipid layer is that it is fluid in nature and not a solid structure. So, the portions of the
membrane move from one point to another point along the surface of the cell.
The materials dissolved in the lipid layer also move to all areas of the cell membrane.
Major lipids are 1. Phospholipids
2. Cholesterol.
Phospholipids
● Phospholipids are lipid substances containing phosphorus and fatty acids. Aminophospholipids, sphingomyelins,
phosphatidylcholine, phosphatidylethanolamine, phosphatidylglycerol, phosphatidylserine and
phosphatidylinositol are the phospholipids present in the lipid layer of the cell membrane.
● The outer part of the phospholipid molecule is called the head portion and the inner portion is called the tail
portion.
● The head portion is the polar end and it is soluble in water and has a strong affinity for water (hydrophilic). The
tail portion is the non-polar end. It is insoluble in water and repelled by water (hydrophobic)
● Two layers of phospholipids are arranged in such a way that the hydrophobic tail portions meet in the center of
the membrane. Hydrophilic head portions of the outer layer face the ECF and those of the inner layer face the
ICF (cytoplasm).
Cholesterol
● Cholesterol molecules are arranged in between the phospholipid molecules.
● Phospholipids are soft so cholesterol ”stiffens” the plasma membrane, making it less fluid and less permeable.
FUNCTION OF LIPIDS MEMBRANE
● The lipid layer of the cell membrane is a semipermeable membrane and allows only the fat-soluble substances to
pass through it.
● Thus, the fat-soluble substances like oxygen, carbon dioxide, and alcohol can pass through this lipid layer.
● The water-soluble substances such as glucose, urea, and electrolytes cannot pass through this layer
2. Protein membrane
➔ Protein layers give protection to the central lipid layer
➔ The protein substances present in these layers are mostly glycoproteins
➔ Protein molecules are classified into two categories:
◆ 1. Integral proteins or transmembrane proteins
● Integral or transmembrane proteins are the proteins that pass through the entire thickness
of the cell membrane from one side to the other side.
● These proteins are tightly bound to the cell membrane
◆ 2. Peripheral proteins or peripheral membrane proteins.
● peripheral membrane proteins are the proteins that are partially embedded in the outer and
inner surfaces of the cell membrane and do not penetrate the cell membrane.
● Peripheral proteins are loosely bound with integral proteins or the lipid layer of the cell
membrane. So, these protein molecules dissociate readily from the cell membrane.
FUNCTIONS OF PROTEIN MEMBRANE
● Integral proteins provide the structural integrity of the cell membrane.
● Channel proteins help in the diffusion of water-soluble substances like glucose.
● Carrier or transport proteins help in the transport of substances across the cell membrane by means of active or
passive transport
● Receptor proteins serve as the receptor sites for hormones and neurotransmitters
● Enzymes: Some of the protein molecules form the enzymes and control chemicals (metabolic) reactions within the
cell membrane
● Pump: Some carrier proteins act as pumps, by which ions are transported actively across the cell membrane
● Cell adhesion molecules or the integral proteins are responsible for the attachment of cells to their neighbors
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